ARM: omap3: ti_omap3_common: Fix CONFIG_SYS_NS16550_REG_SIZE compiler warning

ARM: omap3: ti_omap3_common: Fix CONFIG_SYS_NS16550_REG_SIZE compiler warning

This commit fixes the following compiler warnings when DM_SERIAL is
enabled.

---8<-------------------------------------------------------------------
...

  CC      spl/board/ti/evm/evm.o
In file included from /solution/scm/u-boot-master/board/ti/evm/evm.c:16:0:
/solution/scm/u-boot-master/include/ns16550.h:31:0: warning: "CONFIG_SYS_NS16550_REG_SIZE" redefined
 #define CONFIG_SYS_NS16550_REG_SIZE (-1)

In file included from /solution/scm/u-boot-master/include/configs/omap3_evm.h:22:0,
                 from include/config.h:5,
                 from /solution/scm/u-boot-master/include/common.h:21,
                 from /solution/scm/u-boot-master/board/ti/evm/evm.c:14:
/solution/scm/u-boot-master/include/configs/ti_omap3_common.h:33:0: note: this is the location of the previous definition
 # define CONFIG_SYS_NS16550_REG_SIZE (-4)

  LD      spl/board/ti/evm/built-in.o

...

  CC      spl/drivers/serial/ns16550.o
In file included from /solution/scm/u-boot-master/drivers/serial/ns16550.c:11:0:
/solution/scm/u-boot-master/include/ns16550.h:31:0: warning: "CONFIG_SYS_NS16550_REG_SIZE" redefined
 #define CONFIG_SYS_NS16550_REG_SIZE (-1)

In file included from /solution/scm/u-boot-master/include/configs/omap3_evm.h:22:0,
                 from include/config.h:5,
                 from /solution/scm/u-boot-master/include/common.h:21,
                 from /solution/scm/u-boot-master/drivers/serial/ns16550.c:7:
/solution/scm/u-boot-master/include/configs/ti_omap3_common.h:33:0: note: this is the location of the previous definition
 # define CONFIG_SYS_NS16550_REG_SIZE (-4)

  LD      spl/drivers/serial/built-in.o

...
---8<-------------------------------------------------------------------

Signed-off-by: Derald D. Woods <woods.technical@gmail.com>
Reviewed-by: Tom Rini <trini@konsulko.com>